I work for the State of Florida as a Park Ranger. The manatee is a large female and probably died from a boat strike. Most of these deaths are caused by poor boating skills by people in rental boats! Did she have any babies around her? They just gave birth to new calfs last month.
You might watch for small ones in the area for the next week or so!
Manatees have been here for thousnads of years, and the Spanish did not bring them over here. They were already here when Columbus arrived, and that is when he started the legend of the mermaid. Poor manatee. Yay Sirenia.